不同离子对TC4钛合金电化学腐蚀行为的协同作用机制

图1 TC4钛合金显微组织的OM像

Fig.1 OM images of TC4 alloy with low (a) and high (b) magnifications

2.2 H⁺ 和F^-对TC4钛合金电化学行为的影响

TC4钛合金在含有不同浓度H⁺的3.5%NaCl中的极化曲线和EIS结果如图2所示。极化曲线拟合结果见表1。从图2a及表1拟合结果可以看出，在不含H⁺的3.5%NaCl溶液中，阳极保持钝化趋势，在添加H⁺后仍然表现为自钝化特征；随着H⁺浓度增加，钝化电流密度(i_pp)略有增加，表明H⁺的存在增大了钝化膜阳极溶解的速率，但增大作用非常微弱。极化曲线的阴极分支发生明显变化，在添加H⁺后，阴极还原反应速率增加约1个数量级。从含100 mmol/L H⁺的阴极反应可看出，当阴极极化到较负电位后，有明显氧扩散控制的氧还原反应特征，这表明阴极极化初期电位较正时，主要发生的是氧还原反应，随着电位负移，氧还原反应加快，氧扩散成为控制步骤，在阴极电位达到析氢过电位时，开始发生析氢反应。阴极反应大幅增加的原因包括2个方面，一方面，H⁺的加入使溶液pH值降低，钛合金表面发生阴极还原反应的动力增大；另一方面，H⁺对钝化膜有轻微溶解作用，导致钝化膜对阴极反应电子传输的阻挡作用减弱，最终使阴极还原反应速率大幅增加。TC4钛合金在含不同浓度H⁺的3.5%NaCl中的EIS如图2b所示。可以看出，Nyquist曲线均表现出容抗特征，且容抗弧尺寸并不相同。在未添加H⁺的3.5%NaCl中的容抗弧尺寸最大，添加H⁺后容抗弧尺寸减小，这表明钝化膜的阻挡能力在含H⁺的溶液中略有下降，但不同浓度H⁺的容抗弧尺寸差异不大。

图2

图2 TC4钛合金在含不同浓度H⁺的3.5%NaCl中的极化曲线和EIS结果

Fig.2 Potentiodynamic polarazation curves (a) and EIS (b) of TC4 alloy in 3.5%NaCl solutions with different H⁺ concentrations (c_H)

表1 TC4钛合金在含不同浓度H⁺的3.5%NaCl中极化曲线的拟合结果

Table 1 Fitting results of polarization curves of TC4 alloy in 3.5%NaCl solutions with different c_H

c_H	E_corr	i_corr	b_c	i_pp
mmol·L^-1	mV	μA·cm^-2	mV·decade^-1	μA·cm^-2
0	-502	0.06	-223	4.48
25	-469	1.62	-143	4.50
50	-469	3.14	-159	4.51
100	-494	4.39	-204	4.51

Note:E_corr—corrosion potential, i_corr—corrosion current density, b_c—cathodic Tafel slope, i_pp—passive current density

TC4钛合金在含有不同浓度F^-的3.5%NaCl中的极化曲线和EIS结果如图3所示，极化曲线拟合结果如表2所示。从图3a及表2拟合结果可见，在含微量F^-的中性NaCl中，阳极区钛合金都表现为自钝化行为，但随着F^-浓度增加，阳极的i_pp随之增大，增幅比在含H⁺溶液中更明显，尤其在含25 mmol/L F^-时，i_pp增加一个数量级以上，表现出了从钝化态转变为活化态的趋势，这表明钝化膜的溶解速率随着F^-浓度的增加而逐渐加快，这与文献[19]结果一致。但添加F^-后对极化曲线的阴极影响比较小，都是发生氧还原反应，且反应速率差异不大。如图3b所示，在不同F^-浓度的3.5%NaCl溶液中，TC4钛合金的Nyquist曲线形状都是相似的，呈现明显的容抗特征，随着F^-浓度增加，容抗弧尺寸逐渐减小，表明耐蚀性能下降。前期研究^[17]结果表明，F^-可以参与到钝化膜的溶解反应过程中，随着F^-浓度的增加，钝化膜的溶解反应将加快，但钝化膜未完全遭到破坏时，仍能保持钝化特征。

图3

图3 TC4钛合金在含不同浓度F^-的3.5%NaCl中的极化曲线和EIS结果

Fig.3 Potentiodynamic polarazation curves (a) and EIS (b) of TC4 alloy in 3.5%NaCl solutions with different F^- concentrations (c_F)

表2 TC4钛合金在含不同浓度F^-的3.5%NaCl中极化曲线的拟合结果

Table 2 Fitting results of polarization curves of TC4 alloy in 3.5%NaCl solutions with different c_F

c_F	E_corr	i_corr	b_c	i_pp
mmol·L^-1	mV	μA·cm^-2	mV·decade^-1	μA·cm^-2
0	-502	0.06	-223	4.48
3	-500	0.09	-207	7.65
4	-558	0.12	-166	9.72
6	-648	0.19	-175	14.52
8	-734	0.10	-169	20.97
12	-627	0.20	-268	20.97
25	-744	0.13	-168	63.46

2.3 H⁺ 和F^-对TC4钛合金电化学行为的协同影响

TC4钛合金在含不同浓度F^-的酸性3.5%NaCl + 50 mmol/L H⁺溶液中的极化曲线和EIS结果如图4所示，极化曲线拟合结果见表3。从图4a及表3拟合结果可以看出，当c_F = 0 mmol/L时，TC4钛合金表现为自钝化行为；当c_F ≥ 3 mmol/L后，TC4钛合金表现出活化-钝化行为，这表明钝化膜处于不稳定状态。同时发现，随着F^-浓度的增大，i_pp逐渐增大，这表明钝化膜的阳极溶解速率增大，膜层的稳定性下降。此外，阴极分支也发生变化，当c_F = 0 mmol/L时，阴极反应为氧还原反应和析氢反应共同控制，当c_F ≥ 3 mmol/L后，阴极反应将由析氢反应控制，且析氢反应并不随着F^-浓度的改变而发生变化，这与前文献[20]结果一致，F^-的存在不影响钛合金的阴极反应过程。TC4钛合金在含不同浓度F^-的酸性3.5%NaCl + 50 mmol/L H⁺中的Nyqusit图如图4b所示。当c_F = 0 mmol/L时，Nyqusit图表现为一个单一的容抗弧特征，当c_F ≥ 3 mmol/L后，Nyqusit图上出现了2个明显的容抗弧，且随着F^-浓度增加，容抗弧半径逐渐减小，这说明钝化膜的致密性下降，耐蚀能力降低。

图4

图4 TC4钛合金在含不同浓度F^-的酸性3.5%NaCl + 50 mmol/L H⁺中的极化曲线和EIS结果

Fig.4 Potentiodynamic polarazation curves (a) and EIS (b) of TC4 alloy in 3.5%NaCl + 50 mmol/L H⁺ sol-utions with different c_F (Inset in Fig.4b shows the EIS of TC4 alloy in 3.5%NaCl + 50 mmol/L H⁺ solutions with c_F of 3, 4, 6, and 8 mmol·L^-1)

表3 TC4钛合金在含不同浓度F^-的酸性3.5%NaCl + 50 mmol/L H⁺中极化曲线的拟合结果

Table 3 Fitting results of polarization curves of TC4 alloy in 3.5%NaCl + 50 mmol/L H⁺ solutions with different c_F

c_F	E_corr	i_corr	b_c	i_pp
mmol·L^-1	mV	μA·cm^-2	mV·decade^-1	μA·cm^-2
0	-469	3.14	-159	4.48
3	-892	64.00	-113	89.20
4	-908	78.27	-94	113.50
6	-916	130.43	-104	113.50
8	-955	189.57	-90	239.90

此外，对比了TC4钛合金在F^-浓度相同而H⁺浓度不同的3.5%NaCl溶液中的极化曲线，如图5所示。当H⁺浓度从c_H = 0 mmol/L增加到c_H = 50 mmol/L时，极化曲线的阳极分支从完全的钝化行为转变为活化-钝化行为，同时i_pp增加一个数量级以上，自腐蚀电位下降约400 mV，阴极的反应速率也大幅增加，这表明在含F^-的3.5%NaCl溶液中，增加H ⁺ 浓度使阴阳极反应速率都明显加快。

图5

图5 TC4钛合金在含不同浓度H⁺的3.5%NaCl + 3 mmol/L F^-溶液中的极化曲线

Fig.5 Potentiodynamic polarazation curves of TC4 alloy in 3.5%NaCl + 3 mmol/L F^- solutions with different c_H

对比TC4钛合金在含有不同浓度F^-或H⁺溶液中的极化曲线(图2~5)可以看出，在不同的c_F条件下(3、4、6、8 mmol/L F^-)，增加H⁺后阳极钝化行为转变为了活化-钝化行为，且在相同c_F条件下(3 mmol/L F^-)，增加H⁺后阴阳极反应速率都大幅增加。根据前面的分析，F^-主要影响TC4钛合金表面的阳极反应，使得表面的钝化性能下降，整体阳极溶解速率逐渐增大(表现为阳极曲线的右移)，而对阴极反应没有影响；H⁺主要影响TC4钛合金表面的阴极反应，使阴极反应从氧还原反应转变成主要为析氢反应，大大加快了阴极反应速率。当F^-和H⁺共同作用时，阳极分支右移的距离远远高于F^-单独作用时，因此可以推测：在溶液pH值降低后(添加H⁺)，F^-对钝化膜的作用机制发生改变。王政彬^[20]根据HF的反应常数推测，pH < 3后，溶液中的F^-大部分以HF的形式存在。本实验所用的含50 mmol/L H⁺的溶液pH值为1.3左右，溶液中同时存在HF和F^-。根据文献[21]，HF提供的还原环境会抑制钛合金表面氧化膜重新钝化的速率，并且HF对膜层的破坏作用远远高于F^-，因此，钝化膜的阳极反应速率将会因HF的存在而大大加快。但本工作中添加的是微量的F^-和H⁺，阳极曲线仍然处于钝化态，样品表面形貌没有明显变化，只是失去了金属光泽，颜色略显发黄。

2.4 Fe³⁺ 在酸性F^-中的缓蚀作用机理

TC4钛合金在含不同浓度Fe³⁺的酸性含氟溶液3.5%NaCl + 50 mmol/L H⁺ + 3 mmol/L F^-中的极化曲线和EIS结果如图6所示，极化曲线拟合结果见表4。从图6a及表4拟合结果可以看出，在不含Fe³⁺ (c_Fe = 0 mmol/L)或c_Fe < 2 mmol/L的溶液中，阳极曲线呈现活化-钝化行为，表明此时的钝化膜稳定性较差，溶液中的腐蚀性离子可以穿过钝化膜到达基体；而c_Fe ≥ 2 mmol/L后，阳极曲线表现出自钝化特征，且随着c_Fe的增加，阳极曲线逐渐向左移动，表明钝化膜溶解速率下降。对于阴极分支，在c_Fe = 0和c_Fe = 1 mmol/L时，阴极还原反应为析氢反应控制；而c_Fe ≥ 2 mmol/L后，阴极反应发生改变。此时，溶液中可能发生的阴极反应有3种：氧还原反应、析氢反应和Fe³⁺的还原反应。在c_Fe ≥ 2 mmol/L后，腐蚀电位未达到析氢反应的过电位，因此不考虑析氢反应，只有氧还原反应和Fe³⁺的还原反应能够发生。当c_Fe = 2 mmol/L时，由于c_Fe较低，氧还原反应和Fe³⁺的还原反应速率相差不大，此时阴极反应由氧还原反应和Fe³⁺的还原反应共同控制。当c_Fe > 2 mmol/L时，阴极曲线发生明显的上移，表明此时阴极反应再次发生变化。由于c_Fe增大，Fe³⁺还原反应动力增大，此时阴极为Fe³⁺的还原反应控制。图6b为TC4钛合金在含不同浓度Fe³⁺的酸性含氟溶液中的Nyquist曲线。可知，在c_Fe = 0 mmol/L和c_Fe =1 mmol/L时，Nyquist曲线表现为2个容抗弧，而c_Fe ≥ 2 mmol/L后，Nyquist曲线表现为单一的容抗弧，且随c_Fe增大，容抗弧尺寸逐渐增大，这表明在加入Fe³⁺后膜层的钝化性能增加，且阻挡腐蚀性介质的能力逐渐加强。

图6

图6 TC4钛合金在含不同浓度Fe³⁺的酸性含氟溶液3.5%NaCl + 50 mmol/L H⁺ + 3 mmol/L F^-中的极化曲线和EIS结果

Fig.6 Potentiodynamic polarazation curves (a) and EIS (b) of TC4 alloy in 3.5%NaCl + 50 mmol/L H⁺ + 3 mmol/L F^- solutions with different Fe³⁺ concen-trations (c_Fe)

表4 TC4钛合金在含不同浓度Fe³⁺的酸性含氟溶液3.5%NaCl + 50 mmol/L H⁺ + 3 mmol/L F^-中极化曲线的拟合结果

Table 4 Fitting results of polarization curves of TC4 alloy in 3.5%NaCl + 50 mmol/L H⁺ + 3 mmol/L F^- solutions with different c_Fe

c_Fe	E_corr	i_corr	b_c	i_pp
mmol·L^-1	mV	μA·cm^-2	mV·decade^-1	μA·cm^-2
0	-892	64.00	-113	89.2
1	-892	64.00	-113	72.4
2	-450	9.57	-121	43.1
3	-84	8.68	-179	30.0
4	-43	7.83	-179	23.6
5	11	2.64	-206	18.2

为了进一步探讨Fe³⁺对TC4钛合金阴、阳极曲线的影响，通过测试添加Fe³⁺前后TC4钛合金在3.5%NaCl中单独的阴、阳极曲线，分析其对阳极分支与阴极分支的影响，结果如图7所示。在加入5 mmol/L Fe³⁺后，阴极曲线(图7a)向上迁移，表明阴极反应大幅加速：未添加Fe³⁺时，阴极发生氧还原反应；添加5 mmol/L Fe³⁺后，阴极增加了Fe³⁺的还原反应，且Fe³⁺还原反应的速率远远高于氧还原反应。对于阳极分支(图7b)，除腐蚀电位因阴极反应变化导致的正移外，阳极的钝化区域完全重合，这表明此条件下，Fe³⁺的加入对TC4钛合金的钝化能力没有影响。

图7

图7 Fe³⁺对TC4钛合金在3.5%NaCl中阴极和阳极极化曲线的影响

Fig.7 Effects of Fe³⁺ on the cathodic (a) and anodic (b) polarazation curves of TC4 alloy in 3.5%NaCl solution

3 分析与讨论

F^-与Ti有强烈的络合作用^[22]，可以通过与Ti表面反应破坏钝化膜^[23]。在中性含F^-溶液中，随着F^-浓度的增加，表面钝化膜的阳极溶解速率增大，直观表现为阳极钝化电流密度的右移，相应示意图如图8a所示。

图8

图8 不同离子对钛合金极化曲线影响示意图

Fig.8 Sketch maps of the effect of different ions on the polarazation curves of Ti alloy

(a) F^- (b) H⁺ + F^- (c) H⁺ + F^-+ Fe³⁺

在添加H⁺的酸性溶液中，参与阴极的反应通常有氧还原和析氢2种：

O_{2} + 4 e^{-} + 2 H_{2} O = 4 O H^{-}

(1)

2 H^{+} + 2 e^{-} = H_{2}

(2)

通过Nernst方程可以计算出2个反应的平衡电位(E_e)与溶液中H⁺浓度(pH值)的关系：

E_{e 1} = E_{e (O H^{-} / O_{2})} = 1.299 - 0.0591 p H

(3)

E_{e 2} = E_{e (H_{2} / H^{+})} = - 0.0591 p H

(4)

根据上述公式，氧还原反应的平衡电位始终在析氢反应的平衡电位之上。通常情况下，钛合金阴极发生氧还原反应。然而，当溶液中存在H⁺后，随着H⁺浓度增加(pH值减小)，E_e2将增加，析氢反应的驱动力增加，析氢反应将不能忽略。

在F^-和H⁺同时存在的介质中，HF会导致钝化膜溶解加快。因此，酸性氟溶液中钝化膜的溶解速率将远大于中性氟溶液。阳极曲线右移程度大大增加，相应的示意图如图8b所示。

缓蚀剂Fe³⁺对钛合金在酸性F^-中的电化学过程影响分为2个方面。其一，介质中能参与阴极还原反应的离子增加，除上述的氧还原和析氢反应外，Fe³⁺/Fe²⁺氧化还原对也参与到阴极反应中：

F e^{3 +} + e^{-} = F e^{2 +}

(5)

其平衡电位可以表示为：

E_{e 3} = E_{e (F e^{3 +} / F e^{2 +})} = E^{0} + \frac{R T}{F} l n \frac{[F e^{3 +}]}{[F e^{2 +}]}

(6)

式中，E⁰为标准电位，只与温度和压力有关；R为理想气体常数，8.314 J/(K·mol)；T为热力学温度，K；F为Faraday常数，96485 C/mol；[Fe³⁺]和[Fe²⁺]分别表示Fe³⁺和Fe²⁺的浓度，mol/L。Fe³⁺/Fe²⁺氧化还原对具有快速反应动力和高的平衡电位^[24]。同时，根据Fe(OH)₃的溶解积K_sp：

K_{s p} = [F e^{3 +}] [O H^{-}]^{3}

(7)

在25℃时，根据手册数据^[25]可知，K_sp为3 × 10^-39。根据H₂O的离子积常数K_w，当[H⁺] = 100 mmol/L时，OH^-的浓度[OH^-] = 10^-13。计算得出，加入的Fe³⁺将全部以离子形态存在。也就是说，随着加入Fe³⁺浓度的增加，[Fe³⁺]逐渐增大。Liu等^[16]通过计算得出：25℃条件下，在[Fe³⁺]和[Fe²⁺]皆为1 mmol/L时E_e3的值大约为0.56 V。该平衡电位远远大于E_e2的值([H⁺] = 100 mmol/L时为-0.059 V)。根据式(6)，E_e3将随着[Fe³⁺]的增加而逐渐增大，基于混合电位理论，阴极曲线将逐渐向上移动，相应的示意图如图8c所示。从热力学上分析， $E_{(F e^{3 +} / F e^{2 +})}^{0}$ 的标准电极电位很正(0.77 V），Fe³⁺的还原反应可能使自腐蚀电位大幅正移。另外，电位差ΔE_e3随着Δln[Fe³⁺]线性增大，而与Δ[Fe³⁺]呈对数增长关系，即随[Fe³⁺]增加，ln[Fe³⁺]的增加程度减缓，E_e3的增加逐渐减慢。当[Fe³⁺]增加到一定值后，阴极曲线将不再发生变化，这与图6a得到的结果一致。此时，若阳极曲线不发生改变，阴极曲线的上移可以使得阴阳极曲线的交点从活化-钝化区到达钝化区，也就表现为钛合金的钝化。其二，钛合金的阳极反应也随着[Fe³⁺]的增加发生变化。由于Fe³⁺与F^-非常容易发生络合反应生成[FeF₅]^2- (Fe³⁺与F^-的络合物稳定常数lgβ₅ = 15.77，TiO²⁺与F^-的络合物稳定常数lgβ₃ = 13.7)，因此Fe³⁺的加入将会减弱F^-对钛合金钝化膜的破坏作用。最终，随着[Fe³⁺]的增加，阳极溶解速率降低，表现为阳极曲线的左移，相应的示意图如图8c所示。另外，在不含F^-的溶液中，Fe³⁺只作用于钛合金的阴极部分，而对阳极分支没有影响(图7)，这也侧面论证了上述关于Fe³⁺在含氟酸性溶液中对阳极分支的作用。

4 结论

(1) H⁺的存在对TC4钛合金阳极钝化行为影响较小，主要起到加速阴极反应的作用；F^-的存在对阳极钝化行为影响较大，随着F^-浓度增加，TC4钛合金表面钝化膜的溶解速率逐渐增加，但F^-对阴极还原反应影响较小；当F^-和H⁺共同存在时2者协同作用，大大加速了钝化膜的阳极溶解速率及阴极还原反应速率。

(2) 基于混合电位理论，提出了Fe³⁺对钛合金在酸性F^-中的缓蚀机理。随着Fe³⁺的浓度增加，Fe³⁺的阴极还原反应加速，使阴、阳极曲线的交点从活化-钝化区转移到钝化区；同时，溶液中Fe³⁺通过络合作用降低游离态F^-的浓度，减弱F^-对钝化膜的破坏作用，最终，2者协同影响大大降低了钝化膜的溶解速率。

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